High-pressure compact arc lamp with electrodes containing tantalum carbide

ABSTRACT

A high-pressure compact arc lamp with an envelope which is substantially spherically bulbous where the discharge occurs, and having opposed aligned electrodes therein spaced from each other a distance less than the distance of the bulb wall therefrom at a plane transverse to and between the ends of said electrodes, both of the electrodes comprising sintered bodies composed of from 20 to 90 percent by weight of tantalum carbide and from 80 to 10 percent by weight of at least one metal from the group of tungsten, rhenium, molybdenum and tantalum.

United States Patent [72] Inventors Manfred Rehmet;

Jiirgen Begemann, both of Munich, Germany 10/1955 21 Appl. No. 854,452

7/1959 Anderson,Jr..........,..... 2,965,790 12/1960 Ittigetal........,

VA 1 l 3 3 l. 3

[22] Filed Sept. 2, 1969 [45] Patented Nov. 16,1971

3,061,756 10/1962 Henderson.

2/1965 Rexer.........

Patent-Treuhand-Gesellschaft Fuer Elektrischi Gluehlampen mbH Munich, Germany [32] Priority Sept. 12, 1968 [73] Assignee 2/1966 Hill et a1. 10/1968 Johansen etal. Primary Examiner-Roy Lake Assistant Examiner-Palmer C. Demeo Att0rney- Howard P. King Germany [31] P17649615 [54] HIGH-PRESSURE COMPACT ARC LAMP WITH ELECTRODES CONTAINING TANTALUM CARBXDE 8 Claims, 1 Drawing Fig.

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2,304,412 12/1942 Kern et a1. 313/184 X llllllllllll PATENTEUNUV 16 mm Manfred Reh et Ju'rgen Begemann INVEN TORS ATTORNEY HIGH-PRESSURE COMPACT ARC LAMP WllTllI ELECTRODES CONTAINING TANTALUM CARBIDE GENERAL CONSIDERATIONS The lamp of this invention is one which obtains a brightness from 20 to 200 kilostilbs and is used as an almost fixed point source of light in optical devices. As most generally used, mercury vapor is provided in the bulbous envelope as the gaseous medium for the discharge, and attains a pressure value in operation of from to 100 atmospheres.

It is immediately called to attention that the lamp of the present invention must be distinguished from heretofore known mercury vapor lamps used for street and hanger lighting having approximately tubular envelopes, a wall-stabilized arc of a length exceeding the envelope diameter, and producing a considerably lower brightness than the lamp herein described.

THE PROBLEM Due to the application of the lamp of the present invention in optical devices, it is requisite that instability of the arc of our super high-pressure mercury vapor lamp shall be as low as possible and that the optical quality of the envelope wall during normal lifetime of the lamp is not notably impaired by THE INVENTION We have discovered that the above-mentioned requirements are achieved by utilization of electrodes that are selfsupporting sintered bodies composed of from to 90 percent by weight of tantalum carbide intermixed and sintered with from 80 to 10 percent by weight of at least one of the metals selected from the group of tungsten, rhenium, molybdenum and tantalum. The electrodes taper to smaller ends directed toward each other and spaced apart a distance less than the diameter of the bulbous portion of the envelope at the region thereof opposite the gap between the electrodes. Dischargeinitiating coils with included emitter material therein are provided behind the larger ends ofsaid electrodes.

THE DRAWING Referring to the accompanying drawing, the single FIG. thereof shows a super high pressure compact arc lamp, mostly in longitudinal section, and with the opposite end bases separated therefrom.

DESCRIPTION In the specific embodiment of the invention illustrated in said drawing, an envelope 1 fabricated from bubble-free and scale-free quartz glass in tubular form, an intermediate portion whereof is dilated to constitute approximately a spherical or somewhat ovate discharge space or chamber 2. Longitu dinally of and coaxial with said tubular envelope and projecting toward each other into said discharge space or chamber 2 are metallic rod electrode supports 3 and 4. These rods are appropriately made of ground swaged tungsten and conveniently have a diameter of 1.5 mm. Within said chamber 2 on the forward ends of said rods 3 and t, securely fixed thereon, are electrode bodies 5 and 6 respectively, which by way ofspecific example as a highly acceptable optimum composition, comprise a mixture of 60 percent of tantalum carbide and 40 percent of tungsten, and initially the mixture contains a suitable binder such as 2 percent camphor. The electrode bodies are sintered into a cohesive mass and during the sintering operation the binder evaporates.

The rear end portion of each sintered body 5 and 6 has a cylindrical formation whereas the forward end of each where the arc strikes tapers with-a conical or frustoconical shape. Said electrode bodies do not intentionally contain any emitter additive, although in operation of the lamp small traces in trifling amount may deposit thereon from migration.

ln the manufacture of the electrode bodies 5 and 6 as possible limits of ingredients, a mixture is made from 20 to percent by weight of powdered tantalum carbide stirred with 80 to l0 percent by weight of at least one metal in powdered condition selected from the group of tungsten, rhenium, molybdenum and tantalum, said mixture also initially containing a solvent such as camphor dissolved in diethyl ether. The stirring of the mixture continues with accompanying evaporation of the binder. The electrode bodies are formed from the mixture and sintered, effecting complete evaporation of the binder from the formed blanks. In this: formation of the blanks, the same are subjected to an adequate pressure in the range of 300 to 320 kilograms. Also during this forming of the blanks, a blind-end longitudinal core hole is provided at the cylindrical end thereof for reception of the blank on the end of the sup porting rod, thereby constituting the blank a complete and mounted electrode body. For presintering the blanks, they are annealed for 10 minutes at 1500" C. in a forming gas. if desired, tapering or forming the frustoconical end on the electrode body may be done after sintering. Mounting of the electrode body on the support rod may appropriately be accomplished by shrink-fitting in an atmosphere of forming gas.

Immediately behind each cylindrical end of the electrode body on the respective supporting rod is a starting electrode coil 7 and 8 respectively. Each starting coil is composed of tungsten wire of 0.1 mm. diameter with one inner series of convolutions, preferably four, tightly wound upon the supporting rod, and around said inner series of convolutions is an outer series of three convolutions back-wound in overlying relation to the first series and radially spaced therefrom a distance of 0.1 mm. The outer series of outer convolutions is formed as an integral continuation of the wire forming the inner series. Emitter material, such as thoria, is included between the two series of convolutions and functions solely as a starting aid for the discharge. However, due mainly to the surface migration, some thorium gets into the region of the electrode body where the arc strikes. The emitter material is prepared as a paste and injected between the series of inner.

and outer convolutions of the coil, which may be done if desired before the coil is applied to the electrode rod.

One supporting rod, for instance the upper one, as shown in the drawing, is provided with a gettering coil 9 of tantalum wire of0.5 mm. in diameter which is slipped onto the rod 3 behind the coil 7 on that rod.

At the longitudinally outward end of the supporting rods 3,4 are in-leads I0 and 11 respectively which have the glass of the envelope sealed therearound and at the outer ends thereof bases l2, 13 are provided appropriately connected with said in-leads. A mirror coating 14 is applied both to the annular zone on the bulb wall contiguous to the more constricted tubular portion ofthe envelope so as to be somewhat rearwardly of one of the electrode bodies, and to the tipped-off, bulged in exhaust tip.

The gaseous contents for the bulbous portion of the envelope, where the discharge occurs, comprises about 35 Torrs (pressure unit of mm. of mercury) of argon gas and mercury vapor from just sufficient liquid mercury to completely vaporize.

The dimensions of the electrodes and the power input of the lamp are so coordinated that in the region where the arc strikes a small fusing area forms on each electrode on which thereafter the arc generally strikes without contraction in a stable manner. It is assumed that the high stability of the arc is promoted by the constancy of the work function across the fusing area, probably also by an extremely low work function. The amazingly little blackening of the lamp envelope in spite of the presence of the two fusing areas, especially when using electrodes of sintered material consisting of up to 60 percent by weight of tantalum carbide and to 40 percent by weight of tungsten, is to be understood from the soft," i.e. not contractedly striking are which covers practically the entire fusing area uniformly. By this means too high local heating does not take place in spite of the high electrode load which is necessar-y for the maintenance of a stable arc operation, said local heating otherwise being observed near the spot where the arc strikes in case of contractedly striking arc and thus substantially contributing to evaporation and sputtering of the electrode material, respectively, and thereby to a blackening of the discharge envelope. An increase of the tantalum carbide component leads to increasing blackening, whereas an increase of the tungsten component promotes the formation of individual fusing beads which cause instability of arc.

While the invention has been described in conjunction with a super high pressure lamp containing besides the starting gas merely mercury, the electrodes offer advantages also in case of high-pressure compact arc lamps operated from AC of the type which contain in addition to the mercury, or as the sole light-generating filling, a metal halide. The characteristics of compact are Xenon lamps for AC operation are likewise improved by sintered electrodes of tungsten and tantalum carbide.

We claim:

1. A high-pressure compact arc lamp comprising an elongated tubular envelope having an intermediate bulbous portion, two electrode supports extending longitudinally of the tubular envelope and projecting toward each other into said bulbous portion of the envelope and terminating therein, as arcstriking electrodes, said electrodes being spaced close together with a gap therebetween less that the diameter of said bulbous portion of the envelope around said gap, and said electrodes being constituted as sintered bodies of a mixture of 50 to 70 percent by weight of tantalum carbide and 50 to 30 percent by weight of tungsten.

2. A lamp in accordance with claim I, wherein said bulbous portion of the envelope contains mercury vaporized in operation of the lamp and producing pressure from 10 up to 100 atmospheres.

3. A lamp in accordance with claim 1, wherein the bulbous portion of the envelope contains a metal halide for supplying gas filling therefor.

4. A lamp in accordance with claim 1, wherein said sintered bodies constituting the electrodes are composed of approximately 60 percent by weight of tantalum carbide and approximately 40 percent by weight of tungsten.

5. A lamp in accordance with claim 1, wherein the dimensions of the electrode bodies and the power input are so coordinated that in the region where the arc strikes a small fusing area forms.

6. A lamp in accordance with claim 1, wherein said electrodes have cylindrical ends most proximate to said supports and have their other ends tapered where directed toward each other.

7. A lamp in accordance with claim 1, wherein electrode coils are located on said supports behind said electrode bodies, said electrode coils having an emitter carried between convolutions thereof.

8. A lamp in accordance with claim 7, wherein a gettering coil of tantalum is provided on one of said supports behind the respective sintered electrode body on that support. 

2. A lamp in accordance with claim 1, wherein said bulbous portion of the envelope contains mercury vaporized in operation of the lamp and producing pressure from 10 up to 100 atmospheres.
 3. A lamp in accordance with claim 1, wherein the bulbous portion of the envelope contains a metal halide for supplying gas filling therefor.
 4. A lamp in accordance with claim 1, wherein said sintered bodies constituting the electrodes are composed of approximately 60 percent by weight of tantalum carbide and approximately 40 percent by weight of tungsten.
 5. A lamp in accordance with claim 1, wherein the dimensions of the electrode bodies and the power input are so coordinated that in the region where the arc strikes a small fusing area forms.
 6. A lamp in accordance with claim 1, wherein said electrodes have cylindrical ends most proximate to said supports and have their other ends tapered where directed toward each other.
 7. A lamp in accordance with claim 1, wherein electrode coils are located on said supports behind said electrode bodies, said electrode coils having an emitter carried between convolutions thereof.
 8. A lamp in accordance with claim 7, wherein a gettering coil of tantalum is provided on one of said supports behind the respective sintered electrode body on that support. 